An image sensor typically comprises a two-dimensional array of photo detector circuits. A photo detector circuit may operate as follows. The photo detector circuit comprises a photosensitive device, such as, for example, a photodiode that provides a photo current in response to incident light. The photo current charges or discharges a capacitive node during a given time interval. At the end of this time interval, a photo detection voltage is present on the capacitive node, which represents the incident light. A front-end circuit receives the photo detection voltage and, in response, provides an output signal that is representative of this voltage. The output signal may then be processed so as to obtain, for example, a pixel value.
The article entitled “A 0.5 V Sub-Microwatt CMOS Image Sensor with Pulse-Width Modulation Read-Out” by Scott Hanson et al., published in the IEEE Journal of Solid-State Circuits, Vol. 45, No. 4, April 2010, describes an image sensor with pulse-width modulation read-out. In this image sensor, a pixel structure comprises a two-transistor comparator. This comparator constitutes a front-end circuit, which is coupled to a photo detector circuit as described hereinbefore.